1. Field of the Invention
The invention is concerned with low-tack and tack-free adhesives that substantially do not experience adhesion build-up and so provide tapes that are repositionable, even after being adhered to a substrate for prolonged periods of time. Of special interest are low-tack pressure-sensitive adhesive note pads and adhesive diaper closures.
2. Description of the Related Art
POST-IT.TM. note pads have become virtually indispensable in the office, each sheet bearing a stripe of pressure-sensitive adhesive that is sufficiently tacky to adhere to paper and other substrates but does not show a buildup of adhesion. The sheets can be cleanly removed and reapplied a number of times without loss of tackiness. These attributes are realized due to the use of tacky microspheres coated onto the paper substrate, as disclosed in U.S. Pat. No. 3,691,140 (Silver). The same low-tack pressure-sensitive adhesive has been used for a number of other purposes, e.g. as a coating for a bulletin board or for posting ordinary pieces of paper.
The widespread success of the POST-IT.TM. note pads and the relatively high cost of the adhesive have resulted in efforts to produce a pressure-sensitive adhesive of a more simple, and hence more economical, composition that would provide the same attributes. However, it is believed that every known pressure-sensitive adhesive has been deficient in some important respect, e.g., being too aggressively tacky or lacking the desired initial holding power or being subject to a gradual buildup in adhesion.
An adhesive that is said to afford similar properties as that used in POST-IT.TM. note pads is the subject of U.S. Pat. No. 4,684,685 (Shuman et al.). Shuman et al.'s preferred adhesive includes a natural rubber latex and a tackifier which is an anionic aqueous dispersion of hydrogenated rosin or rosin esters. After coating the dispersion onto a substrate such as a sheet of paper, the dried coating allegedly has enough tack to adhere to solid contact surfaces by manually pressing the coated substrate to that surface. The Shuman patent says that the user can then "reapply the coated substrate as many as eight to ten additional times to the same or another free solid surface."
Although there have been attempts at providing alternative adhesive formulations which provide properties suitable for use as a repositionable adhesive, such as for repositionable note pads, there is still great interest in the discovery of yet more alternatives.
Of additional interest for repositional low-tack adhesives, and otherwise, are adhesives which adhere to themselves yet are subsequently removable and re-adherable. Adhesives with these properties may or may not have repositionable tape or note pad properties. Advantageously, for certain applications these self-adhering adhesives will be relatively non-tacky or have no tack so that they exhibit adhesiveness to little else other than itself. Examples of such adhesives are discussed in U.K. Pat. No. GB 2,116,253B (Clerici et al. ) and U.S. Pat. No. 4,522,874 (Pommez).
Clerici et al. releasably joins two objects using two pieces of "adhesive" tape that can be repeatedly engaged and disengaged. They state that this requires each of (a) the cohesive strength of the carriers, (b) the anchoring of the adhesive layers to the carriers, and (c) the cohesive strength of the adhesive layers to be stronger than the force required to separate two engaged tapes. The Clerici et al. adhesive can be coated from "an elastomer for example natural rubber dissolved in a solvent such as heptane in the proportions of 15% natural rubber and 55% of heptane, by weight" (sentence bridging pages 8 and 9). Also useful is "a mixture of elastomeric materials, consisting for example of natural rubber and elastomers of a synthetic nature, such as butadiene-styrene rubber" (page 4, lines 17-24). The Clerici et al. "adhesive" is just rubber or a mixture of natural and synthetic rubber which is attached to an, e.g., cloth backing.
Pommez illustrates a three-layer disposable diaper, the outer layer of which is "a porous paper-like substrate having one surface thereof coated with a selective adhesive which adheres only to itself" (col. 2, lines 38-41). Referring to FIG. 2: "Ears, or projections 54, of the back portion of the diaper, are also impregnated with the adhesive of the invention from inside, which enables its selective adhesion in any position on the outer surface, also impregnated, of the front portion 56, of the diaper. The qualities of the invention's adhesive enable the fastening and refastening of ear 54 until a perfect fit on the wearer's body is obtained, or permits the diaper to be removed and used again later" (col. 4, lines 1-11). "The adhesive is comprised of an aqueous ammoniacal emulsion having about 60 percent solids and about 0.003 percent ammonia. The solids are about 85 parts by weight poly-cis-isoprene and about 15 parts by weight vinyl acetate and n-butyl acrylate" (col. 2, lines 41- 45).
This emulsion of Pommez serves as an adhesive and as the means by which the outer shell (paper) is made water impermeable. There is no indication that this emulsion would be suitable for use in a tape or on a diaper which is not a porous paper substrate, e.g. a polyolefin backing which would severely limit the applicability of this self-adhering adhesive. Further use of a solvent based system is hampered by the need to subsequently remove and properly dispose of the solvent. This problem is particularly acute where the solvent is a volatile organic, as in Clerici et al.
The present invention has as a general object to solve or substantially alleviate the above noted problems in the art.
Another object of the invention is to provide an adhesive composition that is self-adhering.
A more specific object is to provide a self-adhering adhesive that can be subsequently removed and reapplied to itself.
A more specific object is to provide a self-adhering adhesive which is suitable for use as a repositionable diaper closure.
Another more specific object is to provide a self-adhering adhesive that has low or no adhesive buildup to itself.
Another object is to provide a repositionable adhesive.
A more specific object is to provide a repositionable adhesive which is adhesive to itself.
A further object of the present invention is to provide a method for applying these adhesives without the use of a solvent.
Further advantages and features of the invention, as well as the scope, nature and utilization of the invention will become apparent to those skilled in the art from the following description of the preferred embodiments of the invention.